Basic characteristics required for an electrophotographic photoreceptor include (1) the characteristic that it can be electrified to appropriate potential in a dark place, (2) the characteristic that the electric charge is substantially not dissipated and lost in a dark place, and (3) the characteristic that the electric charge can be rapidly dissipated and lost by irradiating it with light.
Inorganic substances such as selenium, cadmium sulfide, and zinc oxide that have been used to date have many merits, but, at the same time, various deficiencies. For example, selenium, now in wide use, sufficiently meets the above-mentioned conditions (1) to (3), but it has defects in that conditions for preparing it are difficult, it has a high production cost, it is not flexible, it is difficult to process into a belt shape, and it is very sensitive to thermal or mechanical shock so that it must be handled carefully. Cadmium sulfide or zinc oxide is dispersed in a resin as a binder and is used for an electrophotographic photoreceptor, but the phoptoreceptor has deficiencies in mechanical characteristics such as evenness, hardness, tensile strength, friction resistance, and the like, so that it is impossible to use it repeatedly as such.
In recent years, electrophotographic photoreceptors using various organic substances have been proposed to overcome the above-mentioned deficiencies of photoreceptors comprising inorganic substances, and some of them have been put into practice. Examples include an electrophotographic photoreceptor comprising poly-N-vinylcarbazole and 2,4,7-trinitrofluoren-9-one as described in U.S. Pat. No. 3,484,237, an electrophotographic photoreceptor comprising poly-N-vinylcarbazole sensitized with a pyrylium salt-based dye as described in Japanese Patent Publication No. 25658/73, an electrophotographic photoreceptor with an organic pigment base as described in Japanese Patent Application (OPI) No. 37543/72 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), and an electrophotographic photoreceptor having an eutectic complex comprising a dye and a resin as the main component as described in Japanese Patent Application (OPI) No. 10785/72.
In addition, an electrophotographic photoreceptor having copper phthalocyanine dispersed in a resin has also been described, particularly in Japanese Patent Publication No. 1667/77.
These organic electrophotographic photoreceptors have mechanical chracteristics and flexibility improved to some extent as compared with the above-mentioned inorganic electrophotographic photoreceptors, but they have, in general, low photosensitivity and are not so suitable for extended repeated use, so that they are not fully satisfactory electrophotographic photoreceptors.
The photoconduction process of electrophotographic photoreceptors comprises:
(1) a process for generating electric charges through exposure, and PA0 (2) a process for transporting electric charges.
As an example of electrophotographic photoreceptors in which the processes (1) and (2) are carried out by the same substance, there may be mentioned a selenium photosensitive plate. On the other hand, as an example of electrophotographic photoreceptors in which the processes (1) and (2) are each carried out by a different substance, a combination of amorphous selenium and poly-N-vinylcarbazole is well known. A function-separation type electrophotographic photoreceptor in which the processes (1) and (2) are each carried out by a different substance has advantages in that the range for selection of material can be extended, and consequently electrophotographic characteristics such as sensitivity and reception potential of the electrophotographic photoreceptor are improved, and substances suitable for preparation of coating film of electrophotographic photoreceptor can be selected from an extensive range.
Many function-separation type electrophotographic photoreceptors like this have been proposed, but only a few of these have been put into practice, and even those that have been put into practice have some deficiencies.
It has been proposed to select an electric charge transporting substance to be combined with an electric charge generating substance by taking the ionization potential as a standard, but such is a result of studies on electrophotographic photoreceptors prepared by selection of combination of specific materials of the same kind, so that it lacks generality and it cannot clearly account for electrophotographic characteristics of an electrophotographic photoreceptor prepared by selection between materials of different kinds.
The fact is that the combination of an electric charge generating substance with an electric charge transporting substance is studied largely by the method of trial and error at the present time. For example, with respect to electrophotographic photoreceptors that are characterized by an electric charge transporting substance, there have been made proposals described in Japanese Patent Application (OPI) Nos. 186847/85 (corresponding to U.S. Pat. No. 4,594,304), 196767/85 (corresponding to U.S. application Ser. No. 713,720), 262162/85 (corresponding to U.S. Pat. No. 4,619,880), and 35365/87 (corresponding to U.S. application Ser. No. 894,534). However, these proposed electrophotographic photoreceptors are still unsatisfactory in terms of sensitivity in a long wavelength region and, hence, realization of photoreceptor having a higher sensitivity has been demanded.